GDS has some tips:
"Only collect phone numbers from people if you really need them. Not everyone has or can use a phone, so be sure to give users the option to contact them.
Allow different formats
Users should be allowed to enter phone numbers in any format that is familiar to them. It must allow spaces, hyphens, square brackets and additional hyphens, and be able to accommodate country and area codes.
Validate phone numbers
You must validate the phone numbers so that users can know if they entered one incorrectly. Google's libphonenumber library can validate phone numbers in most countries.
If the phone number is not in the correct format and there is no example
Say "Enter a phone number, such as 01632 960 001, 07700 900 982 or +44 0808 157 0192".
If the phone number is not in the correct format and there is an example
Say "Enter a phone number in the correct format."
Make it clear what type of phone number you need
Use the form label or the text of the suggestion to tell users if you specifically need a UK, international or mobile phone number.
If you want to include a sample phone number (in the text of the suggestion, for example), Ofcom maintains a list of numbers reserved for use in the media. These are:
Non-geographic United Kingdom: 01632 960000 to 960999
United Kingdom London: 020 7946 0000 to 7946 0999
United Kingdom mobile: 07700 900000 to 900999
Explain why you need a phone number.
Tell users why you could contact them and when.
Do not show phone numbers as links on devices that cannot make calls
It is possible to dial phone numbers as links, like this:
020 7947 6330
However, in doing so, phone numbers will be designed as links, which is confusing on devices that do not support phone calls, such as most desktop machines.
Nor is it necessary: most modern mobile browsers automatically detect phone numbers and display them as links anyway.
If you need to mark your phone number as links, for example, to support a device that cannot detect them automatically, make sure they are not displayed as links on devices that cannot make calls.
Enter phone numbers in the GOV.UK style
Avoid entry masking
Avoid entry masking because it makes it difficult for users:
write a number in your preferred form
transcribe a number from another place and verify that they have succeeded
Avoid reformatting phone numbers
The GOV.UK notification team has observed that some users get confused when they are presented with a reformatted version of a phone number that they provided, for example, with the country code +44 added.
Research on this pattern
More research is needed on the best way to handle: