Strip invalid characters from phone number

the characters "() -" are used for formatting a phone number.  But
ModemManager works only with plain numbers without any formatting.

Only characters used for formatting numbers are stripped.
3 jobs for master in 3 minutes and 29 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #282693
librem5
build-debian-gcc-buster

00:00:59

 
  Package
passed #282694
librem5
package:deb-debian-buster

00:00:46

passed #282695
librem5:arm64
package:deb-debian-buster:arm64

00:02:29