Hi Blelisa welcome
See this is where my laid back attitude kicks in. I just love it when people say things to me like " well I think having a heading like phone systems on your website is totally unprofessional, no one is going to take you seriously"
Me I use what brings the bacon home for my customers, I don't care what the business snobs say, I just do the research, then build the site around that research, and while they sneer at the use of downmarket phrases like 'phone system' choosing to use 'telephone systems' on their own site, YOUR customer can also be laughing, all the way to the bank.
Please tell me what is wrong with talking to your potential customers in a language they choose to use? if your prospects want phone systems, GIVE THEM phone systems as a primary phrase, and drop in telephone systems as a secondary.
You say 'because it sounds more professional' but to whom does it sound unprofessional? Certainly not the people who searched for the phrase and found what they are looking for! to them you are talking their language.