Add tertiary phone number

[jensschuppe]

... just as #52 did for a secondary phone number, i. e. a phone3 parameter and configuration for the phone type.

[jensschuppe]

@jaapjansma fancy reviewing this, as you authored the secondary phone thing?

This could also use a test which I haven't had the time adding.

[bjendres]

Looks good to me, but I haven't tested.

I'm fairly sure it needs to be "tertiary" instead of "ternary", just so we don't confuse English native speakers. But if you don't want to change it, i'll add a "sic" :)

Let me know if you need/want me to actually test it.

[jensschuppe]

Let me know if you need/want me to actually test it.

@bjendres There's a unit test for secondary phone which might be easily adjusted oder copied for testing phone3, feel free to add one if you like.

[bjendres]

Let me know if you need/want me to actually test it.

@bjendres There's a unit test for secondary phone which might be easily adjusted oder copied for testing phone3, feel free to add one if you like.

I did, but I cannot assess whether this copy tests all the right things. But the tests all pass now.