can't hear iphone ringer

[Dave D]

Hi all, received an iPhone 4s last Tuesday as the main communication device at work ( formerly had Telus PTT mike phones) first Apple product I've had,love it.I work in the oil field trucking business (constant noise,wear hearing protection) Other operators and myself are now missing most calls cause iPhone's ringer is maybe half as loud as the mic phones were (volumes all the way up and vibrate to) So is there a extra loud high pitch ring tone available that might solve the problem ?(doesn't have to be cute,something like a backup beeper for heavy equipment would be fine) Or what other solution might you recommend ?
thanks