Here are a bunch of ideas for teaching about 'On the Telephone'. Activities are organized under different headings, but some can be used at various stages of a lesson, and adapted to other lessons too. Choose the activities that you feel suit you and, most importantly, your students best. Arrange them in a lesson plan as you see fit alongside your own activities. Be sure to give students as much help with the target expressions/language as they need. Feel free to improve the example handouts as much as you like.
 
 
Greeting/Introduction Ideas

JTE asks ALT about their keitai, how often they use it, do they speak in Japanese on the phone etc.
ALT and JTE have a telephone conversation in front of the class. Then check students' understanding. Can be funny/serious as you like.
 
 
Warm-ups/Vocab Activities

Common Expressions/Memory:
Give students a handout with common English expressions used on the telephone. Have the JTE write the same expressions on the board in Japanese. Students try to match the English with the Japanese, either individually, or in teams. You could also write out both the English and Japanese on large pieces of paper and, using lots of magnets, play a big, team game of memory, where teams take turns turning over 1 English and 1 Japanese expression, getting points if they make a match. Alternatively, memory could be played on a smaller scale within teams. This kind of activity is good for having students try to work out the meaning of expressions, without having been 'taught' them yet.
 
Telephone/Chinese Whispers:
Students play Chinese whispers in rows as teams. The ALT tells an expression to the back students who then send the message along the rows to the front students who tell the JTE. All messages are expressions that are commonly used on the telephone, e.g. "May I speak to…?" and "Can I take a message?" After each expression, the ALT can write it on the blackboard, explain it and give alternatives, before moving on to the next expression. If done in order, eventually a basic dialog made from all the expressions could be written on the blackboard ready for class practice and the next activity.
 
Telephone Numbers:
The ALT quickly shouts out telephone numbers, which the students listen to and try to understand. How understanding is expressed is up to you. Students could simply try to write the numbers down and then the ALT/JTE gives the answer on the blackboard. Or students could make teams. Each team gets a set of number cards and when the ALT shouts out the number, teams have to line up with each student holding a number card, in the correct order.
 
 
Practice Activities 
 
Take a Message:
Give students something like this example handout and practice the dialog with them, helping with new expressions etc. When the students are ready, have them walk around the class and have different phone conversations with different people. With each partner, students practice giving and receiving messages, writing down memos at the same time. Students are learning a real English life-skill.
 

Consolidation/Filler Activities 
 
Talk with the ALT:
Basically, you are looking for a handful of students to one by one have a telephone conversation with the ALT in front of everyone at the end of the class. This works in some classes, but not in others. There are various ways to choose which students: Ask for volunteers (does happen in some genki classes - is great when it does); Pick some at your discretion; Have the JTE choose some; Call out class numbers; Play hot potato or do any other kind of lottery system. Make sure to encourage a huge round of applause when a student has finished. You can choose to incorporate mobile phones, real or fake, into the activity if you like.
  
Email the ALT:
This may or may not be something you want to do, but you could ask the students to send you one mail each on your mobile phone, introducing themselves. Some enthusiastic students will be happy to. Some students won't be interested. It's up to you if you want to give out your mail address and also whether you want to reply, which could be expensive, but it's an idea.
 

Extra Expressions:
Teach students expressions that you commonly use on the phone with your friends (slang etc.). Ask the students to teach you the Japanese equivalent.
 

Advanced Activities

Skits:
Students work in twos or threes to make a short skit of their own using the standard dialog, but with freedom to change it if they like. Have some pairs/groups act out their skit.
 
 
Easier Activities

With less-advanced classes, it might be better to spend more time working on the basic expressions, including the telephone and memory games, before have the students practice the dialog in pairs. Use a similar handout as the above main activity, but with just two memo boxes.