I've just tried calling a prospective customer through Hubspot calling, and got a "Phone number is invalid" message (see attached). The phone number is correct, but it would appear that Hubspot is using a check on the number of digits. While the standard for UK numbers is to have a 5-digit area code, prefixed "0" (i.e. 01234), followed by a 6-digit phone number, there are still a large number of 5-digit phone numbers which are valid (such as this one).
I'd suggest this check is removed. If the user intended to enter a 6-digit number, and mis-typed, the call should be allowed to fail. Garbage in, garbage out. This kind of validation is annoying and unhelpful!