If you have a mobile wallet, do you lose your coins when you lose your phone? I'm new to this Bitcoin stuff and discussing it with friends. Somebody said, if you "have bitcoins on your phone" and you lose your phone, you also lose your bitcoins.

Is this true?
No, not necessarily. In fact, if you actually follow the instructions given by most wallet software, you won't lose your Bitcoin.

When you first setup a Bitcoin wallet, most modern wallet software will walk you through making backups of your wallet (which are not stored on your phone). Most will also force you to click through multiple dialogs of "I understand that not backing up means I can lose all of my coins" if you try to skip the backup process. If you have a backup, then you can restore your wallet on a new phone or on a computer with a different wallet software. If you do not have a backup, then yes, you will lose all of your Bitcoin.

Alternatively you could use a web wallet where the information necessary to spend your coins are stored on the internet with a third party. In this case, if you lose your phone, you won't lose your coins so long as you remember your login credentials. But I wouldn't recommend using a web wallet as they can cause many other security and privacy issues that can result in you losing all of your coins.