For SMS messages, the caller ID must be set to an SMS-enabled phone number on your account. Tropo supports alphanumeric sender IDs, but these must be configured at your account instead of set inside your application. See Understanding Sender ID for details on this feature.
For voice calls, laws in the US and other countries often restrict what caller ID you can send, as a method of reducing misleading calls, spam, and other abusive uses. Additionally, some phone networks will behave unpredictably if completely invalid phone numbers or formats are used. Here are some tips to ensure that your caller ID is delivered properly.
- Only set the caller ID to phone numbers that you have permission to use. Setting the caller ID to someone's number without their permission can violate the law.
- Avoid setting the callerID to the same number you are calling. Setting it so it looks like you're calling a number from itself will yield unpredictable results. Some mobile carriers will send the call straight to voicemail. Some carriers will drop the call. Others will ignore the defined caller ID entirely.
- Use a real phone number when setting callerID. While Tropo will allow you to set callerID to a single digit or even a letter, it's not recommended. Your call will be rejected or will be delivered with an unpredictable caller ID if the caller ID you set is something that can't be a real phone number. Examples of this are using non-existent country and area codes, dialing codes or prefixes that don't match the area code, or something other than numerals.
- In TV and movies, US phone numbers are often expressed with a 555 prefix. 555 is a special prefix that is reserved for use by the phone companies, so movies know that any numbers using 555 are always fictitious and cannot be called; many of our examples show 555 phone numbers for this same reason. It also isn't recommended to use a +1555 number as your caller ID, however, as it too can cause unpredictable results. We've even seen a carrier strip one of the 5s and send the caller ID as +1407551212, apparently just for fun.
- There are some numbers that Tropo will not allow you to set as callerID. These include numbers for emergency services (like the US's 911 or 999 in the UK) and numbers that may be used for illegal purposes.