If someone has got your phone number can they hack into your phone using just the phone number? Perhaps by sending an SMS?
This question is too broad and too naive for this forum.
If anyone that knows a phone number can hack into that phone, then we would have giant botnets. Think about it:
for phone = 0 to 99999999 send malicious sms to 06-phone
(06 is the Dutch prefix for mobile phones, if you're wondering.)
So, most malicious SMS messages will involve some form of social engineering (Hi, I'm a Nigerian prince, please install my app) or even just call you (Hi, we're the microsoft helpdesk, can you install something on your computer)
If you're serious about looking into SMS security, look at "Messaging Attacks on Android: Vulnerabilities and Intrusion Detection" by Khodor Hamandi, Alaa Salman e.a.